Subminiature fuses, as with other types of fuses, are used to protect circuit components from damage that can be caused by excess current flowing through the circuit Excess current is generally categorized as either an overload current or a short circuit current. Overload current is generally considered to be in the range of 135 percent to 200 percent of normal ("rated") current. Short circuit current may be up to 1000 percent of rated current.
Conventional subminiature fuses run the risk of catastrophic failure during interruption of short circuit currents associated with high voltage (on the order of 250 Volts), and high ampere applications with a lower power factor. The circuit power factor relates to the inductive component of the load. The reduction of the power factor is associated with an increase in inductance of the circuit.
The voltage generated by the inductive component is related to the current rate by the equation: ##EQU1## Where: L is the inductance; i is the current; V is the voltage; and t is time. As is evident from the equation because of the short interruption time of the fast clearing fuse, the rate of current drop is often large relative to a slower clearing fuse. Thus it is possible to measure voltage across a fast clearing fuse that is three to four times higher than the RMS (Root Mean Squared) value of the nominal voltage of the AC circuit. Lower power factor circuits will result in higher voltage across the fuse. Higher voltage across the fuse means greater possibility of breakdown in the fuse.
Among other factors, the industry standard for fuse performance is based on three factors: (1) the maximum continuous current (the "current rating"); (2) the "peak current", and (3) I.sup.2 t value associated with short circuit performance of the fuse at rated voltage. The current rating is the amount of current the fuse can sustain, continuously, without opening. The peak current is the maximum value of the current that downstream components will experience before the fuse interrupts the circuit. The I.sup.2 t value is a measure of energy that the fuse will allow downstream components to experience during the interrupt event The purpose of the fuse protection is to minimize both the peak current and the I.sup.2 t value while at the same time, maintain the desired current rating.
There are two components of I.sup.2 t. The first component is "melting I.sup.2 t" and it corresponds to the amount of energy the fuse will pass to downstream components before the starting of the arc. The second component is "arcing I.sup.2 t" and it corresponds to the amount of energy that the fuse will pass to downstream components during the arcing period after the fuse element has melted.
To minimize the I.sup.2 t value and the peak current, the circuit interrupt (or "clearing of the fuse element") must occur as quickly as possible. In the normal fuse, the fuse element cross section is determined by the current rating desired For a given fuse element material, to obtain a higher fuse rating, larger cross sectional area for the fuse element is required to pass the current without overheating. Unfortunately, a larger cross sectional fuse element means that there is more material to melt. For a given overload level, larger cross sectional area for the fuse element will take longer to melt than smaller cross sectional area fuse element. Longer melting time is undesirable, because it results in larger I.sup.2 t values and higher peak current values. Larger cross sectional area of the fuse element also means there is more material and a larger amount of current supplied for the subsequent arc, which also adds to the overall I.sup.2 t value, further decreasing fuse performance. In the past, fuse performance had to be sacrificed to gain a higher fuse current rating
Many conventional fuses are constructed from a fuse element and a two piece fuse housing comprising a cap and a base. During a short circuit condition, pressure inside the housing increases. Due to the small physical size of the subminiature fuse and hence the short arc clearing gap, the housing for such a fuse is subject to catastrophic failure problems that are not normally inherent in a physically larger fuse. There is a risk that the subminiature fuse housing will blow apart or rupture. In the two piece housing design, this normally occurs at the seal between the cap and the base. If the housing ruptures, this would not only expose a live arc but would also prolong that arc, thereby potentially causing damage to circuit components downstream of the fuse due to the additional time required to fully interrupt the circuit. Once the housing begins to leak, the pressure in the housing begins to decrease This causes the interruption time to increase.
Those skilled in the art know that when the fuse element is subjected to short circuit current, the fuse element heats up until it reaches the melting point of the fuse element conductor. The rate of the heat build up is, among other things, a function of the magnitude of the excess current. Once the temperature of the conductor reaches its melting point, the conductor material rapidly vaporizes mixing vaporized metal atoms with the gas or air medium surrounding the conductor. Upon vaporization, an arc is formed in the gas mixture which is created by the vaporization of the fuse element. The resulting plasma acts as a conducting path for the arc. The increased temperature of the arc plasma also increases the pressure of the fuse housing. If the arc plasma becomes dense, the travel of the charged particles in the plasma is restricted. Decreased mobility of the charged particles increases the resistance of the gap, thereby acting to extinguish the arc.
There have been several attempts to solve this problem of catastrophic fuse failure in subminiature fuses. One example is illustrated in the U.S. Pat. No. 4,417,226 to Asdollahi, et al. In this patent a ceramic lining is utilized in the interior of a two piece fuse housing to insulate the plastic body from the heat produced during a short circuit condition Merely coating the interior of an air filled fuse housing with a ceramic lining does not provide a fast clearing fuse. The relatively large interior volume of air and a low out-gassing ceramic lining prevents the quick pressure increase required for fast arc clearing. Lower pressure in the fuse housing tends to facilitate charged particle mobility in the plasma. Increased mobility in the plasma results in prolonged arcing. It is the prolonged arcing that generates the heat which jeopardizes the integrity of the fuse housing.
Other prior fuses utilized a ceramic coating around the fuse element. This coating will not work well if there are cracks in the coating or air pockets (voids) surrounding the fuse element. Cracking and voids are a problem with prior fuse designs. Air voids provide additional volume for expansion of the pressurized vapor during the interrupt process, decreasing resistance of the arc and increasing the interrupt time, thus decreasing fuse performance. Cracks in the ceramic coating contributed to premature fuse element overheating and melting. In the prior art fuses, the fuse element assembly was dipped into liquid ceramic material Due to the viscosity and surface tension of the liquid, there is not enough control over the amount or distribution of the material around the fuse assembly. As a result, air voids often remained in the ceramic material. Also, when the ceramic cures and contracts, cracks can form around the fuse element. As discussed above, both air voids and cracks decrease fuse performance.
Other examples of subminiature fuses are embodied in U.S. Pat. Nos. 2,941,059 to Sims, et al. and 3,775,723 to Mamrick, et al. Others have attempted, but have failed, to reduce this risk of catastrophic failure by improving the strength of the fuse housing to prevent rupture thereof.
The most effective fuse to date for this application is embodied in U.S. Pat. No. 4,612,529 to Gurevich et al. Although this fuse is effective for normal applications, it is not effective for 250 V applications above three amperes Other past fuse designs are susceptible to catastrophic failure when they were used in a circuit with a lower power factor (less than 100 percent)
Because of past failures, the need still exists to provide a subminiature fuse capable of properly interrupting short circuit or overload current with a power factor less than 100 percent with minimal risk of catastrophic failure. A higher ampere subminiature fuse device with a short duration of arcing time at low I.sup.2 t values would be well received by the industry.